How to Measure a Roof for Shingles Safely and Accurately

The most accurate way to measure your roof is to divide it into sections. For example, if your roof is a simple gable roof with two planes that lean into one another, measure your roof as two individual sections. You'll add the parts of the roof together later. If you have a hip roof, consider your roof as a combination of separate triangular and trapezoidal planes.

Make a rough drawing of each roof section on a piece of paper. The drawings only need to be a rough outline. Don't worry about drawing them to scale.

From your ladder or from on the roof, individually measure each plane and transfer the measurements to the appropriate spot on your drawings.

• For rectangular sections, measure from the peak to the lower edge (H) and from side to side (W). 

• To measure triangular sections, measure from the peak to the gutter (H) and the width of the bottom edge (W), making notes on your sketch as you go.

• For trapezoidal and other section shapes, you'll need the measurement of the top and bottom edges (W) and the height from the peak to the gutter (H).

Calculate the square-foot areas of each roof section using the following mathematical formulas.

• Find the area of a rectangle by multiplying H by W.

• For triangles, multiply H by W and divide by two.

• For trapezoids, add the top edge width to the bottom edge width, multiply by H, and divide by two.

After determining the square-foot totals for each roof section, add all the sums together to reach the total roof square feet.

When calculating the amount of shingle bundles necessary for a roof, you'll hear the term, squares. A square is simply a unit of measurement equal to 100 square feet. Convert your roof's area into squares by dividing its square-foot measurement by 100.

Like most construction work, installing roof shingles comes with some material waste. Roofing waste happens due to using some shingles for starter strips and cutting in valleys and around obstructions. 

The amount of material waste for roofing work is often between 10% and 20%. To determine the total amount of shingles your roofing project will need, multiply the total number of measured squares by 10% for simple roofs, 20% for complex roofs with many planes or obstructions, or 15% for somewhere in between.

Shingles are available in bundles roughly equal to one-third of a square. To determine how many shingle bundles you need for your roofing job, multiply the total number of squares needed for your roof, including waste, by three and round up to the next whole number.

Keep in mind that roof shingles are often available in batches or lots. Asphalt shingles last long enough that if your roof requires repair work sometime in the next 20 years or so, the exact color may not be available. For this reason, you may wish to grab a few extra bundles to store for later.